Hydrogen Oxidation and Evolution Reactions

While it is expected that electrocatalytic reactions on Ru surfaces should be strongly structure-sensitive, the first report on structural effects on hydrogen oxidation and evolution reactions appeared only recently The structural effects in the hydrogen oxidation reaction (HOR) and the hydrogen evolution reaction (HER) may be factors affecting the performance of hydrogen fuel cell anodes. 

Hydrogen oxidation on Ru is likely to proceed through a mechanism involving two or three of the following reactions, as on 

The kinetics of the HOR on polyctyslallinc Ru and carbon-supported nanoparticles is about two orders-of-magnitude smaller than that on Pt or Pt-Ru alloys, and it usually is assumed that the Ru contribution to the H2 oxidation current of fuel cell anodes is negligible. However, at temperatures at which PEMFCs operate (60-80 °C) the kinetics of HOR on Ru is considerably faster than at room temperature, so that the effect of Ru surfaces may be of importance in PEMFC catalysis. 

The hydrogen oxidation reaction was found to be under kinetic control at both Ru(OOOl) and Ru(lOlO) surfaces. Dependence on the rotation rate is negligible on the (0001) surface, while 

The structural effects on hydrogen evolution kinetics on Ru are small, as inferred from the HER curves for (0001)- and (lOlO)-oriented surfaces obtained in perchloric and sulfuric acid solutions The similarities in the reaction kinetics in the two indicate that hydrogen evolution proceeds on bare Ru surface, i.e., a surface not covered with either OH or, in the case of sulfuric-acid solutions, with bisulfate ions. 

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Skulason E, Tripkovic V, Bjorketun ME, et al. Modeling the Electrochemical Hydrogen Oxidation and Evolution Reactions on the Basis of Density Functional Theory Calculations. J Phys Chem C. 2010 114 18182-97. 

Sheng W, Gasteiger HA, Shao-Hom Y (2010) Hydrogen oxidation and evolution reaction kinetics on platinum acid vs alkaline electrolytes. J Electrochem Soc 157(11) B1529-B1536. doi 10.1149/1.3483106... 

Neyerlin KC, Gu W, Jome J, Gasteiger HA (2007) A study of the exchange current density for the hydrogen oxidation and evolution reactions. 

Rheinlander P, Henning S, Herranz J, Gasteiger HA (2012) Comparing hydrogen oxidation and evolution reaction kinetics on polycrystalline platinum in 0.1 M and 1 M KOH. ECS Trans 50(2) 2163-2174... 

Skulason E, Tripkovic V, Bjorketun ME et ai (2010) Modeling the electrochemical hydrogen oxidation and evolution reactions on the basis of density functional theory calculations. J Phys Chem C 114 22374... 

Hydrogen Oxidation and Evolution on Platinum in Acids, Fig. 2 Typical voltammetry black) and polarization curves for the HER/HOR (Wne) and ORR/OER red) reactions measured, respectively, on carbon-supported Pt nanoparticles in Ar-, H2-, and 02-saturated 0.1 M HCIO4 solutions. The vertical dotted lines show the reversible potentials for the HER/HOR and ORR/OER... 

Similar size effects have been observed in some other electrochemical systems, but by far not in all of them. At platinized platinum, the rate of hydrogen ionization and evolution is approximately an order of magnitude lower than at smooth platinum. Yet in the literature, examples can be found where such a size effect is absent or where it is in the opposite direction. In cathodic oxygen reduction at platinum and at silver, there is little difference in the reaction rates between smooth and disperse electrodes. In methanol oxidation at nickel electrodes in alkaline solution, the reaction rate increases markedly with increasing degree of dispersion of the nickel powders. Such size effects have been reported in many papers and were the subject of reviews (Kinoshita, 1982 Mukerjee, 1990). 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

Table I also shows the great diversity of organisms in which iron—sulfur proteins have been detected. Thus far there is no organism which when appropriately examined has not contained an iron-sulfur protein, either in the soluble or membrane-bound form. Iron-sulfur proteins catalyze reactions of physiological importance in obligate anaerobic bacteria, such as hydrogen uptake and evolution, ATP formation, pyruvate metabolism, nitrogen fixation, and photosynthetic electron transport. These properties and reactions can be considered primitive and thus make iron-sulfur proteins a good place to start the study of evolution. These key reactions are also important in higher organisms. Other reactions catalyzed by iron-sulfur proteins can be added such as hydroxylation, nitrate and nitrite reduction, sulfite reduction, NADH oxidation, xanthine oxidation, and many other reactions (Table II).

Both oxidation and reduction reactions that occur as corrosion proceeds can result in pH changes. The O2 reduction and hydrogen evolution reactions both either consume protons or produce hydroxyls. Therefore, under conditions in which there is a spatial separation of cathodic and anodic reactions, the pH at the cathodic sites will increase. Hydrolysis of metal cations will result in a decrease in pH at anodic sites. Even for uniform corrosion, with distributed and changing anodic and cathodic sites, the pH will tend... 

In this part, complex electrochemical interfaces and electrochemical reactions on surfaces with various molecules in solvents will be discussed. Examples are the oxidation and evolution of hydrogen on different transition metal surfaces, the reduction of oxygen on several surfaces as well as carbon monoxide reactions, and a complex photoactive reaction in a solar cell. 

The situation with respect to secondary reactions is shown in Fig. 1.34. It is similar to that in the nickel/cadmium battery shown in Fig. 1.32 as far as the positive electrode is concerned. Different is the situation at the negative electrode. The electrode potential is nearly the same, since the equilibrium potential of the hydrogen electrode is only about 20 mV below that of the cadmium electrode. But now hydrogen is used as active material instead of cadmium, and hydrogen evolution as well as hydrogen oxidation are fast reactions, since both are catalyzed by the platinum surface of the negative electrode. 

The chemistry of ethyl alcohol is largely that of the hydroxyl group, namely, reactions of dehydration, dehydrogenation, oxidation, and esterification. The hydrogen atom of the hydroxyl group can be replaced by an active metal, such as sodium, potassium, and calcium, to form a metal ethoxide (ethylate) with the evolution of hydrogen gas (see Alkoxides, metal). 

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